Part 5: Snubber Performance May 2011

1. 1
Transformer Failure Due to Circuit Breaker
Induced Switching Transients
Part 5: Snubber Performance Measurements
David D. Shipp, PE
Fellow, IEEE
Eaton Electrical Group
130 Commonwealth Dr.
Warrendale, PA 15086
Thomas J. Dionise, PE
Senior Member, IEEE
Eaton Electrical Group
130 Commonwealth Dr.
Warrendale, PA 15086
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Snubber Performance Measurements
• Data Center
• Purpose of Measurements
• Test procedures
• Medium voltage measurements
• Selecting a meter
• Using voltage dividers
• Safety concerns
• Data Center challenges

2. 2
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Purpose of Performance Measurements
• Ensure the proper operation of the snubbers
• Measure the transient overvoltage waveforms at the transformer
primary
• produced during switching of the primary vacuum circuit breakers
• verify they do not exhibit high frequency transients (magnitude, rate-
of-rise and frequency).
• Specify the meters and voltage dividers needed for the transient
measurement
• guide electrical contractor with their installation
• electrical contractor to operate all equipment following the test
procedure
• After completion of the site monitoring during the snubber tests,
analyze the measurement data and document the test results in
an engineering report.
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Data Center
Highlights
• Tier III
• LEEDS Certified
• 12.5MVA Capacity
• 13.2KV Ring Bus

3. 3
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Data Center – Measurement Locations
Vacuum
Breaker
Short
Cable
Cast-Coil
Transformer
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Data Center – Electrical Highlights
• 2 x 24.9 kV lines from Factory Shoals and Buzzard
Roast
• 2 x 12.5 MVA transformers
• 13.2 kV “ring-bus” configuration
• MSA and MSB and generator bus GPS
• Vacuum circuit breakers – 600A mains & ties, 200A feeders
• 3 x 2250 KW generators
• 6 x 3750KVA cast coil transformers, 90kV BIL
• 3 x DE Subs CSA, CSB and CSC
• 3 x SE Subs MDSA, MDSB and MLB
• Feeder cable lengths vary from 109 to 249 Feet

4. 4
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RC Snubber at MVS - Typical
200 A
Disconnect to
transformerRC Snubber
13.8 kV
0.25 microFarad
30 ohm
Cast Coil
3750KVA
Transformer
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RC Snubber at MVS - Typical
Snubber Specs
13.8 kV
0.25 microFarad
30 ohm

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Test Procedure
• Test Procedure required by contractor
• Prepared 2 weeks in advance of testing
• developed the instructions
• Included safety briefing each day
• Site specifics supplied by the contractor
• LO/TO instructions
• Breaker operations
• All meter connections made de-energized & LO/TO
• No one in transformer room during tests
• Signature of “Responsible Engineer” before test began
• detailed test procedure form
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Measurement Equipment
• Measurement equipment included voltage
dividers and a transient recording device
• Voltage Dividers
• Capacitive and resistive elements
• 10MHz frequency response
• Three-Phase Power Quality Recorder
• transient voltage waveshape sampling
• 200 nsec sample resolution
• 5 Mhz sampling

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Test Measurement Setup (Typical)
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Voltage Divider Connections
• Highly stranded No. 8, 15 kV insulated hookup wire
• Insulated wire was a precaution – could have used
bare conductor
• maintained 8inch minimum separation for 15kV
between phases and ground
• Routed wires with gradual curve – no 90 degree
bends
• Connection at surge arrester at bus to transformer
primary windings

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Voltage Divider Connections
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Voltage Divider Connections
• Base of each divider grounded
• Grounded to ground bus in primary disconnect
• Used same highly stranded no. 8, 15kV insulated
hookup wire
• Connection to PQ Meter
• Cables to match input impedance of meter to that
of voltage divider
• Voltage divider was calibrated prior to shipping to
site

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CT Connections
• Primary interest was voltage at transformer
• Secondary interest was snubber current
• Connected CTs at ground side of snubbers
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Initial view of events
Energize
(see zoom)
De-energize
(see zoom)
• RMS trend of voltage
• VCB energized and de-energized transformer

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Zoom View of Energize
• Slight overshoot above normal crest
• Ring well damped
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Zoom View of De-Energize
• No transient on opening
• Well damped

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Other Tests – transformer natural frequencies
• Sweep Frequency Response Analysis (SFRA)
ImpedancePhaseAngle
Frequency
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Data Center Challenges
• Contractor requested measurements
• Advised contractor
• First hookup/measurement – 1 day
• Remaining 5 transformers – 2nd day
• Prepare two voltage divider carts in advance
• Contractor agreed to schedule but…
• This was a construction site
• Project manager had other objectives
• Snubber testing viewed as an “inconvenience”
• Voltage divider carts were not prepared
• Be prepared to negotiate

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Summary
• Prepare test procedure – 2 weeks in advance
• Request onelines and site photos to plan hookups
• Ship all equipment in advance
• For safety, two engineers perform measurements
• Follow test procedure
• Make all connections de-energized, LO/TO
• No one in transformer room during test
• Radio contact with contractor performing switching
• Download data after each test
• Analyze results before proceeding to next test
• Begin building test report while on-site